Bone fixation devices are useful for promoting the proper healing of injured or damaged vertebral bone segments caused by trauma, tumor growth, or degenerative disc disease. These external fixation devices immobilize the injured bone segments to ensure the proper growth of new osseous tissue between the damaged segments. External bone fixation devices such as these often include internal bracing and instrumentation to stabilize the spinal column to facilitate the efficient healing of the damaged area without deformity or instability, while minimizing any immobilization and post-operative care of the patient.
One type of external bone fixation device is an osteosynthesis plate, more commonly referred to as a bone plate, that can be used to immobilize adjacent skeletal parts such as vertebral bones. Typically, the fixation plate is a rigid metal or polymeric plate positioned to span bones or bone segments that require immobilization with respect to one another. The plate is fastened to the respective bones, using anchors such as bone screws, so that the plate remains in contact with the bones and fixes them in a desired position. Anterior cervical plates, for instance, can be useful in providing the mechanical support necessary to keep vertebral bodies in proper position and bridge a weakened or diseased area such as when a disc, vertebral body or spinal fragment has been removed. These anterior cervical plates usually include a rigid bone plate having a plurality of screw openings. The openings are either holes or slots that allow for freedom of screw movement. The bone plate is placed against the damaged vertebral bodies and bone screws are used to secure the bone plate to the spine, usually with the bone screws being driven into the vertebral bodies.
While current bone plates and bone screws are effective, unintentional loosening of the screws can reduce the effectiveness of an anterior construct and can result in erosion and irritation of the esophagus. Several techniques have been developed to prevent screw back-out, however many current techniques require the use of a second locking mechanism that is applied to the bone screw or plate. This can complicate the procedure, as it requires the use of additional tools as well as proper placement of the additional locking mechanism. Other existing techniques require precise alignment of the bone screw with the thru-hole in the bone plate, thereby limiting the insertion trajectory of the bone screw.
Accordingly, there remains a need for improved methods and devices for locking a bone screw to a bone plate.